In almost all present disk devices, both small and large, a head suspension assembly of the structure as shown in FIG. 4 disclosed in Japanese Published Examined Patent Application No. 58-22827 is used. FIGS. 4(a) and (b) show a perspective view and a side view of a prior art head suspension assembly, respectively. The head suspension assembly 40 consists of a slider 1 having a coil for writing and reading data relative to a magnetic disk 30, a flexure 2 or a gimbal spring for supporting the slider 1, a load beam 3 for holding the flexure 2, and a mounting block 4 for fixing the load beam 3 on the arm. This head suspension assembly 40 is connected to a head actuator arm 9.
The slider is held by the head suspension assembly so that the slider can travel in a stable manner over the disk at a constant distance therefrom. In particular, the slider must be held in operation at a constant distance from the disk regardless of the distortion of the disk surface or external vibrations. If the distance between the slider and the disk cannot be held constant, write failure and/or read error may occur. In the worst case, the head will collide against the disk causing head crash. Once head crash occurs, written data cannot be read any more.
In order to hold the slider at a constant distance from the disk, the load beam of the head suspension assembly is provided with the required level of mechanical properties such as natural frequency and rigidity by bending the edges 6 upward as shown in FIG. 4. That is, the load beam has a structure consisting of a long, narrow plate-like base 5, and edges 6 bent upward from the base 5. Although the height h of the load beam (i.e., the height of the edges 6) depends on the material of the load beam, it cannot be reduced below a certain value, or its mechanical properties will be degraded. The reason is that if the height h is lower than the above certain value, the natural frequency of the load beam decreases, which alters the allowable frequency of operation of the head positioning servo system. Also, the resistance of the load beam against bending weakens, which affects the head access operation.
In magnetic disk devices, particularly in small hard disk devices, increases in both head access speed and storage capacity are required as the improvements are made in the performance of personal computers in which such disk devices are incorporated.
When the head access operation speed is increased, the natural frequency of mechanical parts for such operation, especially of a head suspension load beam must be high; and therefore, the rigidity of each part must be high.
On the other hand, two approaches are employed for increasing storage capacity an increase in the amount of information stored on a disk, and an increase in the number of disks used in a disk device. For the former, the amount of information has been increased to a considerable level by the use of thin film magnetic disks, thin film heads, and MIG (Metal In Gap) heads. For the latter, the external dimensions, especially the height, of disk devices are limited because they are incorporated in personal computers. Therefore, it is difficult to increase the numbers of disks and heads without increasing the height of the disk devices.
In fact, although the thickness of a disk has been decreased by replacing the aluminum substrate with glass, it is impossible to further decrease disk thickness because of problems relating to magnetic properties as well as the mechanical strength of the disk. Also, decrease in the height of a load beam lowers the natural frequency in the primary vibration mode, causing problems in the head access operation.
It is desired, therefore, to find a means to decrease the height of a load beam without degrading its mechanical properties.
The inventor examined means to solve this problem. First, the inventor considered the fabrication of a load beam with a material of high rigidity to decrease its height. However, such a material was difficult to machine precisely, and the high rigidity of the material made it difficult for the slider to travel properly. Also, the use of such a special material led to an increase in production costs. Therefore, desirable effects cannot be expected from the selection of such a material.
Next, experiments to minimize the influence of reducing the height of the load beam on its mechanical properties were carried out by increasing the thickness of the load beam. As the thickness of the load beam was increased, its natural frequency increased. However, the spring constant of the load beam was also increased significantly. The great change in the value of the spring constant affects the traveling characteristics of the slider, and the performance and reliability of the magnetic disk device are significantly lowered. By such an increase in thickness, therefore, the improvement of mechanical properties cannot be expected.
Further experiments were carried out to minimize the influence of reducing the height of the load beam on its mechanical properties by decreasing the length of the load beam. As the length of the load beam was decreased, its natural frequency increased moderately, and the change in the spring constant was small. In order to increase natural frequency to the desired value, however, the length of the load beam must be considerably shortened, resulting in a large increase of the spring constant. Therefore, a decrease in the length of the load beam will not improve its mechanical properties sufficiently.
Since the selection of such materials, increase in thickness, or decrease in length will not reduce the height of the load beam effectively, for example, in a small 3.5 inch hard disk device, the maximum numbers of disks and heads are limited to 4 and 8, respectively, due to the limitation of the height.
Nevertheless, in order to achieve an increase in the storage capacity of a magnetic disk device, the numbers of disks and heads used in a disk device must be increased.